Mike,
This realising we're not immortal seems to happen to many of us as we get older.
That includes me, and I haven't been flying anything like as long as you have. The answer to your question comes down to one thing, and one thing only - do you want to give up? If you do, then fine. There's no law of the universe that says because you've been flying for 40 years you have to carry on; you can take up something else, or just chill out for a while. You can give up for a bit...then the next time the sun shines and you see that little aircraft overhead and wish you were there, you can start up again. Or if those suggestions actually sound awful, then carry on. Maybe it'll be different from how it was before, maybe you'll never get back your carefree attitude of the past, but you can find a different enjoyment, if that's what you want to do. But it doesn't have to be a big decision to be all made at once.